1. Field of the Invention
The invention relates to a transmission for a vehicle which includes a plurality of drive transmission mechanisms, such as gear pairs and the like, in power transmission lines and which performs the shifting of the transmission speed ratio by changing the drive transmission mechanisms involved in the torque transmission. In particular, the invention relates to a transmission capable of using a combination of the power transmission by mechanical means, such as gear mechanisms or the like, and the power transmission via hydraulic pressure of pressure fluid or the like, or another form of energy, such as electric power or the like.
2. Description of the Related Art
An example of the foregoing type of transmission is described in Japanese Patent Application Publication No. 11-51150 (JP-A-11-51150). A transmission described in Japanese Patent Application Publication No. 11-51150 (JP-A-11-51150) is constructed of a mechanical transmission (MT) that switches the torque transmission path in a planetary gear mechanism in accordance with the states of engagement/release of a plurality of clutch mechanisms, and a hydrostatic transmission (HST) that transmits power by supplying pressure oil produced from a hydraulic pump to a hydraulic motor and that performs the shifting, and the two types of transmissions are disposed parallel to each other between an input member and an output member. In this transmission, the speed change ratio set by the mechanical transmission varies stepwise whereas the speed change ratio set by the hydrostatic transmission continuously vary. Therefore, the speed change ratio of the transmission as a whole can be continuously changed, and the transmission can be caused to function as a so-called continuously variable transmission.
Another example is described in Japanese Patent Application Publication No. 2000-320644 (JP-A-2000-320644). In a transmission described in Japanese Patent Application Publication No. 2000-320644 (JP-A-2000-320644), the power output by a motive power source is distributed and transmitted to a multi-step speed change device that is made up mainly of a plurality of gear pairs and a plurality of clutch mechanisms and to an HST (hydrostatic transmission). After the powers changed in speed by the multi-step speed change device and the HST are combined by a planetary gear mechanism, the power is output from the transmission. Therefore, in this transmission, the proportion between the power transmitted by the multi-step speed change device and the power transmitted by the HST is changed through the use of the HST, so that the speed change ratio as a whole can be continuously changed.
As described above, in the transmission described in Japanese Patent Application Publication No. 11-51150 (JP-A-11-51150), power is transmitted via the hydrostatic transmission, and the proportion of the power transmitted via the HST can be changed so as to steplessly change the speed change ratio of the transmission. However, for the transmission of power via fluid in that case, a pump is directly driven by power from the motive power source. Then, the thus-generated fluid pressure is sent to a motor to drive the motor, and the power output by the motor is directly transmitted to the output side. Therefore, there is possibility that as the fluid pressure becomes higher in accordance with the torque transmitted, the power loss may relatively increase, and therefore the power transmission efficiency as a whole may not become sufficiently high.
This is also the case with the transmission described in Japanese Patent Application Publication No. 2000-320644 (JP-A-2000-320644). In the construction described in Japanese Patent Application Publication No. 2000-320644 (JP-A-2000-320644), since the multi-step speed change device and the HST are disposed substantially parallel to each other between the input member and the output member, there is possibility that the power transmission efficiency may not become sufficiently high due to increased power loss or the like when the power transmission via the HST is performed.
Furthermore, in either of the transmissions described in Japanese Patent Application Publication No. 11-51150 (JP-A-11-51150) and Japanese Patent Application Publication No. 2000-320644 (JP-A-2000-320644), clutches, such as multi-plate clutches or the like, are used as mechanisms for inputting power output by a motive power source, such as an engine or the like, to the transmission or for cutting off the power, and therefore power, such as oil pressure or the like, for the clutches is consumed. This may possibly become a factor that increases the power loss as a whole or degrades the power transmission efficiency.